---
layout: docs
page_title: Garbage collection
description: |-
  Nomad garbage collects Access Control List (ACL) tokens, allocations, deployments, encryption root keys, evaluations, jobs, nodes, plugins, and Container Storage Interface (CSI) volumes. Learn about server-side and client-side garbage collection processes, including configuration and triggers.
---

# Garbage collection

Nomad garbage collection is not the same as garbage collection in a programming
language, but the motivation behind its design is similar: garbage collection
frees up memory allocated for objects that the schedular no longer references or
needs. Nomad only garbage collects objects that are in a terminal state and only
after a delay to allow inspection or debugging.

Nomad runs garbage collection processes on servers and on client nodes. You may
also manually trigger garbage collection on the server.

Nomad garbage collects the following objects:

- [ACL token](#configuration)
- [Allocation](#client-side-garbage-collection)
- [CSI Plugin](#configuration)
- [Deployment](#configuration)
- [Encryption root key](#configuration)
- [Evaluation](#configuration)
- [Job](#configuration)
- [Node](#configuration)
- [Volume](#configuration)

## Cascading garbage collection

Nomad's scheduled garbage collection processes generally handle each resource
type independently. However, there is an implicit cascading relationship because
of how objects reference each other. In practice, when Nomad garbage collects a
higher-level object, Nomad also removes the object's associated sub-objects to
prevent orphaned objects.

For example, garbage collecting a job also causes Nomad to drop all of that
job's remaining evaluations, deployments, and allocation records from the state.
Nomad garbage collects those objects, either as part of the job garbage
collection process or by each object's own garbage collection processes running
immediately after. Nomad's scheduled garbage collection processes only garbage
collect objects after they are terminal for at least the specified time
threshold and no longer needed for future scheduling decisions. Note that when
you force garbage collection by running the `nomad system gc` command, Nomad
ignores the specified time threshold.

## Server-side garbage collection

The Nomad server leader starts periodic garbage collection processes that clean
objects marked for garbage collection from memory. Nomad automatically marks
some objects, like evaluations, for garbage collection. Alternatively, you may
manually mark jobs for garbage collection by running `nomad system gc`, which
runs the garbage collection process.

### Configuration

These settings govern garbage collection behavior on the server nodes. You may
review the intervals in the [`config.go`
class](https://github.com/hashicorp/nomad/blob/b11619010e1c83488e14e2785569e515b2769062/nomad/config.go#L564)
for objects without a configurable interval setting.

| Object | Interval | Threshold |
|---|---|---|
| **ACL token** | 5 minutes | [`acl_token_gc_threshold`](/nomad/docs/configuration/server#acl_token_gc_threshold)<br/>Default: 1 hour |
| **CSI Plugin** | 5 minutes | [`csi_plugin_gc_threshold`](/nomad/docs/configuration/server#csi_plugin_gc_threshold)<br/>Default: 1 hour |
| **Deployment** | 5 minutes | [`deployment_gc_threshold`](/nomad/docs/configuration/server#deployment_gc_threshold)<br/>Default: 1 hour |
| **Encryption root key** | [`root_key_gc_interval`](/nomad/docs/configuration/server#root_key_gc_interval)<br/>Default: 10 minutes | [`root_key_gc_threshold`](/nomad/docs/configuration/server#root_key_gc_threshold)<br/>Default: 1 hour  |
| **Evaluation** | 5 minutes | [`eval_gc_threshold`](/nomad/docs/configuration/server#eval_gc_threshold) <br/>Default: 1 hour |
| **Evaluation, batch** |  5 minutes | [`batch_eval_gc_threshold`](/nomad/docs/configuration/server#batch_eval_gc_threshold)<br/>Default: 24 hours |
| **Job** | [`job_gc_interval`](/nomad/docs/configuration/server#job_gc_interval)<br/>Default: 5 minutes | [`job_gc_threshold`](/nomad/docs/configuration/server#job_gc_threshold)<br/>Default: 4 hours |
| **Node** | 5 minutes | [`node_gc_threshold`](/nomad/docs/configuration/server#node_gc_threshold)<br/>Default: 24 hours |
| **Volume** | [`csi_volume_claim_gc_interval`](/nomad/docs/configuration/server#csi_volume_claim_gc_interval)<br/>Default: 5 minutes| [`csi_volume_claim_gc_threshold`](/nomad/docs/configuration/server#csi_volume_claim_gc_threshold)<br/>Default: 1 hour |

### Triggers

The server garbage collection processes wake up at configured intervals to scan
for any expired or terminal objects to permanently delete, provided the object's
time in a terminal state exceeds its garbage collection threshold. For example,
a job's default garbage collection threshold is four hours, so the job must be
in a terminal state for at least four hours before the garbage collection
process permanently deletes the job and its dependent objects.

When you force garbage collection by manually running the `nomad system gc`
command, you are telling the garbage collection process to ignore thresholds and
immediately purge all terminal objects on all servers and clients.

## Client-side garbage collection

On each client node, Nomad must clean up resources from terminated allocations
to free disk and memory on the machine.

### Configuration

These settings govern allocation garbage collection behavior on each client node.

| Parameter | Default | Description  |
| -------- | ------- | ------------- |
| [`gc_interval`](/nomad/docs/configuration/client#gc_interval)  | 1 minute | Interval at which Nomad attempts to garbage collect terminal allocation directories |
| [ `gc_disk_usage_threshold` ](/nomad/docs/configuration/client#gc_disk_usage_threshold)  | 80 | Disk usage percent which Nomad tries to maintain by garbage collecting terminal allocations |
| [ `gc_inode_usage_threshold` ](/nomad/docs/configuration/client#gc_inode_usage_threshold) | 70 | Inode usage percent which Nomad tries to maintain by garbage collecting terminal allocations |
| [ `gc_max_allocs` ](/nomad/docs/configuration/client#gc_max_allocs) | 50 | Maximum number of allocations which a client will track before triggering a garbage collection of terminal allocations |
| [ `gc_parallel_destroys ` ](/nomad/docs/configuration/client#gc_parallel_destroys) | 2 | Maximum number of parallel destroys allowed by the garbage collector |

Refer to the [client block in agent configuration
reference](/nomad/docs/configuration/client) for complete parameter descriptions
and examples.

Note that there is no time-based retention setting for allocations. Unlike jobs
or evaluations, you cannot specify a time to keep allocations alive before
garbage collection. As soon as an allocation is terminal, it becomes eligible
for cleanup if the configured thresholds demand it.

### Triggers

Nomad's client runs allocation garbage collection based on these triggers:

- Scheduled interval

  The garbage collection process launches a ticker based on the configured
  `gc_interval`. On each tick, the garbage collection process checks to see if it needs to remove terminal allocations.

- Terminal state

  When an allocation transitions to a terminal state, Nomad marks
  the allocation for garbage collection and then signals the garbage collection
  process to run immediately.

- Allocation placement

  Nomad may preemptively run garbage collection to make room for new
  allocations. The client garbage collects older, terminal allocations if adding new allocations would exceed the `gc_max_allocs` limit.

- Forced garbage collection

  When you force garbage collection by running the `nomad system gc` command,
  the garbage collection process removes all terminal objects on all servers and
  clients, ignoring thresholds.

Nomad does not continuously monitor disk or inode usage to trigger garbage
collection. Instead, Nomad only checks disk and inode thresholds when one of the
aforementioned triggers invokes the garbage collection process. The
`gc_inode_usage_threshold` and `gc_disk_usage_threshold` values do not trigger
garbage collection; rather, those values influence how the garbage collector
behaves during a collection run.

### Allocation selection

When the garbage collection process runs, Nomad destroys as many finished
allocations as needed to meet the resource thresholds. The client maintains a
priority queue of terminal allocations ordered by the time they were marked
finished, oldest first.

The process repeatedly evicts allocations from the queue until the conditions
are back within configured limits. Specifically, the garbage collection loop
checks, in order:

1. If disk usage exceeds `gc_disk_usage_threshold` value
1. If inode usage exceeds `gc_inode_usage_threshold` value
1. If the count of allocations exceeds `gc_max_allocs` value

If any one of these conditions is true, the garbage collector selects the oldest
finished allocation for removal.

After deleting one allocation, the loop re-checks the metrics and continues
removing the next-oldest allocation until all thresholds are satisfied or
until there are no more terminal allocs. This means in a single run, the
garbage collection removes multiple allocations back-to-back if the node was
far over the limits. The evictions happen in termination-time order, which is
oldest completed allocations first.

If node's usage and allocation count are under the limits, a normal garbage
collection cycle does not remove any allocations. In other words, periodic and
event-driven garbage collection does not delete allocations just because they
are finished. There has to be pressure or a limit reached. The exception is when
an administrative command or server-side removal triggers client-side garbage
collection. Aside from that forced scenario, the default behavior is
threshold-driven: Nomad leaves allocations on disk until it needs to reclaim
those allocations due to space, inode, or count limits being hit.

### Task driver resources garbage collection

Most task drivers do not have their own garbage collection process. When an
allocation is terminal, the client garbage collection process communicates with
the task driver to ensure the task's resources have been cleaned up. Note that
the Docker task driver periodically cleans up its own resources. Refer to the
[Docker task driver plugin
options](https://developer.hashicorp.com/nomad/docs/deploy/task-driver/docker#gc) for
details.

When a task has configured restart attempts and the task fails, the Nomad client
attempts an in-place task restart within the same allocation. The task driver
starts a new process or container for the task. If the task continues to fail
and exceeds the configured restart attempts, Nomad terminates the task and marks
the allocation as terminal. The task driver then cleans up its resources, such
as a Docker container or cgroups. When the garbage collection process runs, it
makes sure that the task driver cleanup is done before deleting the allocation.
If a task driver fails to clean up properly, Nomad logs errors but continues the
garbage collection process. Task driver cleanup failure issues can influence
when the allocation truly frees up. For instance, if volumes are not detached,
disk space might not be fully reclaimed until fixed.

## Resources

- [Nomad's internal garbage collection and optimization discovery during the
  Nomad Bench project blog post](https://www.hashicorp.com/en/blog/nomad-garbage-collection-optimization-discovery-during-nomad-bench)
- Configuration

  - [client Block in Agent Configuration](/nomad/docs/configuration/client)
  - [server Block in Agent Configuration](/nomad/docs/configuration/server)

- [the `nomad system gc` command reference](/nomad/commands/system/gc)
- [System HTTP API Force GC](/nomad/api-docs/system#force-gc)
